The present invention relates to a hydraulic press such as a cold forging press, a plastic forming press and the like comprising a hydraulic cylinder for driving a movable die of a metal mold which has a molding area smaller than an end surface area of a piston of the hydraulic cylinder so that a pressing force of the hydraulic cylinder is concentrated in the small molding area of the mold to press a workpiece under a high pressure.
A conventional example of such a hydraulic press is illustrated in FIG. 1. This conventional hydraulic press comprises a portal frame 1, a hydraulic cylinder 4 and a metal mold 9. The metal mold 9 shown in FIG. 1 has a molding area A smaller than an end surface area B of a piston of the hydraulic cylinder so that a pressing force of the hydraulic cylinder is concentrated in the small molding area of the metal mold to press a workpiece under a high pressure.
It is noted from FIG. 1 that the portal frame 1 is composed of a bed 1a, columns 1b standing at opposite sides on the bed 1a, and a crown 1c extending over the columns 1b. Each of the columns 1b is integrally connected to the bed 1a and crown 1c by means of tie rods 2 and nuts 3 fastened to upper ends of the tie rods.
The hydraulic cylinder 4 has a cylinder tube 4a fitted in a through hole formed in the central portion of the crown 1c, and a piston 4b slidably fitted in the cylindrical bore of the cylinder tube 4a. The piston 4b has a piston rod 4c which is extended out of the cylinder tube 4a through a cylinder lid 4d fixed to the open end of the cylinder tube. The reference numeral 5 denotes an O-ring interposed between the end of the cylinder tube 4a and the lid 4d, 6 oil seals on the piston 4b and 7 an oil seal in the cylinder lid 4d.
The piston rod 4c is connected at its extended lower end to a slider 8 which has long sliding side surfaces in order to resist an eccentric load. The sliding side surfaces slidably contact opposite gibs 1d and 1d secured to the side columns 1b and 1b, respectively.
The metal mold 9 is composed of an upper forming die 9a secured to the lower end of the slider 8 and a lower molding die 9b fixedly mounted on the bed 1a of the frame 1. The molding area (A) of the mold 9 is defined by areas of horizontal surfaces of the upper or lower molding die and greatly smaller than the area (B) of the top end surface of the piston 4b.
The conventional hydraulic press adapted for supporting a high press forming pressure occurring between the dies 9a and 9b during die forming by rigidity of the frame 1 of the hydraulic press is designed to support a reaction of the press forming pressure. This is the feature of the hydraulic press most different from hammer machines, the conventional hydraulic press of high pressure type generally has substantially the same construction as that of a generic type press, but the more the reaction increases the more the frame constituting member becomes large since the great reaction must be supported by the frame 1.
The frame 1 of the conventional hydraulic press shown in FIG. 1 seems too large for the small metal mold 9a, 9b. In the cold forging press, however, a high pressing pressure is required for press forming a small workpiece. For example, a compression force of more than 1,000 tons is required for press forming a 100 mm-diameter steel sheet so that a hydraulic cylinder having about one meter in diameter is required. Accordingly, a hydraulic press using a square metal mold 300 mm in each side length becomes incongruously large. Thus, a specification of the press is not dependent on a size of the metal mold to be used, but in the case shown in FIG. 1, is dependent on a dimension which the hydraulic cylinder 4 and the tie rod nut 3 do not interface. As mentioned above, the conventional hydraulic press is constructed for convenience of the maker rather than the user. However, it is no good that the metal mold securing surface is unnecessarily large since mechanical strength of the press is largely affected. That is, the supporting points are located far from the load as shown in FIG. 1 so that the frame 1 is largely deformed. In order to prevent such a deformation of the frame, it is necessary to increase the rigidity of the frame. Accordingly, in view of the required rigidity mentioned above, conventional hydraulic presses having a capacity more than one thousand tons could not be manufactured it low cost.